Salinity tolerant surfactant oil recovery process

ABSTRACT

Primary anionic surfactants such as organic sulfonates are effective for recovering oil from subterranean formations only if the salinity and/or hardness of the formation water is relatively low. In petroleum formations containing high salinity, or hard water, either the high salinity water must be displaced by a preflush or the primary anionic surfactant must be used in conjunction with an effective solubilizing co-surfactant, such as ethoxylated alcohols or alkyl phenols, or alkyl or alkylaryl thiols, as well as sulfated or sulfonated, ethoxylated alcohols or alkyl phenols. Optimum performance is achieved if two or more samples of petroleum sulfonate having different equivalent weight ranges and distributions are blended in a ratio which requires the minimum amount of solubilizing cosurfactant to achieve a condition of borderline solubility in the particular formation water in which the surfactants are to be employed. This ratio may be identified by preparing a number of blended petroleum sulfonate samples in the formation water using different ratios of total primary surfactant concentration to solubilizing co-surfactant concentration, and determining whether borderline solubility is achieved, by direct observation or by identifying the sample which produced the minimum electrical conductivity, or at which point the conductivity vs. concentration ratio curve exhibits an inflection point.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns an enhanced oil recovery process and morespecifically, a surfactant flooding oil recovery process. Still morespecifically, this invention is concerned with an oil recovery processusable in subterranean oil formations containing water having abnormallyhigh salinities and/or concentrations of divalent ions such as calciumand magnesium by the use of a critical blend of two or more primaryanionic surfactants differing in equivalent weight distribution and oneor more solubilizing co-surfactants.

2. Description of the Prior Art

Petroleum is normally recovered from subterranean formations in which ithas accumulated by penetrating the formation with one or more wells andpumping or permitting the petroleum to flow to the surface through thesewells. Recovery of petroleum from formations is possible only if certainconditions exist in the formation. The petroleum must be present in theformation in an adequately high concentration, and there must besufficient permeability or interconnected flow channels within theformation to permit the flow of fluids therethrough if sufficientpressure is applied to the fluids. When the formation has natural energypresent in the form of an underlying active water drive, or gasdissolved in the petroleum which can exert pressure to drive thepetroleum to the producing well, or a high pressure gas cap above thepetroleum-saturated portion of the formation, this natural energy may beutilized to recover petroleum. Recovery of petroleum by use of naturalenergy as described above is referred to as primary recovery. When thisnatural energy source is depleted, or in those instances where theformation does not contain sufficient natural energy to support primaryrecovery, some form of supplemental oil recovery or enhanced oilrecovery process must be applied to the formation in order to extractpetroleum therefrom. Supplemental recovery is sometimes referred to inthe art as secondary or tertiary recovery, although in fact it may beprimary, secondary or tertiary in sequence of employment.

Water flooding, which involves the injection of water into thesubterranean, petroliferous formation for the purpose of displacingpetroleum toward the producing well, is the most economical and widelypracticed supplemental recovery method. Water does not displacepetroleum with high efficiency, however, since water and oil areimmiscible, and also because the interfacial tension between water andoil is quite high. Persons skilled in the art of oil recovery haverecognized this inherent weakness in water flooding and many additiveshave been described in the literature for decreasing the interfacialtension between the injected water and the formation petroleum. Forexample, U.S. Pat. No. 2,233,381 (1941) discloses the use of polyglycolethers as surface active agents or surfactants to increase the capillarydisplacement efficiency of an aqueous flooding medium. U.S. Pat. No.3,302,713 discloses the use of petroleum sulfonates prepared from aspecific boiling range fraction of the petroleum feed stock as asurfactant in oil recovery operation. Other surfactants which have beenproposed for oil recovery operations include alkylpyridinium salts,alkyl sulfates, alkylaryl sulfates, ethoxylated alkyl or alkylarylsulfates, alkyl sulfonates, alkylaryl sulfonates, and quaternaryammonium salts.

The above described surfactants are satisfactory in formations where thesalinity and/or concentration of divalent ions in the formation water isrelatively low. Generally, the salinity must be less than about 5,000parts per million and the concentration of divalent ions must be lessthan about 200 to about 500 parts per million in order to permit the useof the most commonly available primary anionic surfactants such aspetroleum sulfonate or other organic sulfonates.

Persons skilled in the art have recognized the limitation of singleanionic surfactants such as petroleum sulfonate and have described theuse of certain solubilizing co-surfactants therewith. U.S. Pat. Nos.3,811,504; 3,811,505; and 3,811,507 describe mixtures of alkyl oralkylaryl sulfonates which exhibit satisfactory performance in petroleumformations having high salinity and/or hard water. U.S. Pat. No.3,508,612 (1970) describes the use of a dual surfactant systemcomprising an organic sulfonate such as a petroleum sulfonate and asulfated, ethoxylated primary or secondary alcohol, which is compatiblewith high salinity and/or high divalent ion containing formation waters.U.S. Pat. Nos. 3,827,497 and 3,890,239 relate to oil recovery fluids andprocesses which are compatible with high salinity formation waters andinvolve mixtures containing organic sulfonate and sulfonated,ethoxylated alcohols.

While the aforementioned multi-component systems can be rendered solublein high salinity and/or high divalent ion concentration formationwaters, their use has not always been satisfactory because the ratio ofthe concentrations of the primary anionic surfactant and thesolubilizing co-surfactant is extremely critical and varies with thesalinity, divalent ion concentration, as well as with the specificsurfactants being employed. If too little solubilizing surfactant isused, the primary anionic surfactant precipitates in the presence of thehigh salinity water. If too much solubilizing surfactant is used, thematerial is rendered so soluble in water that its effectiveness forpurpose of reducing the interfacial tension between the drive water andthe formation petroleum is greatly reduced. In either case, oil recoveryfalls off sharply. Moreover, the cost of the solubilizing co-surfactantis generally two to five times as great as the cost per pound of theprimary anionic surfactants and the use of excessive amounts ofsolubilizing co-surfactant renders an oil recovery process economicallyunattractive.

U.S. Pat. No. 3,916,997 (1975) described the use of an oil-externalmicellar dispersion wherein the concentration of surfactant and alcoholused as a solubilizer are varied to produce a fluid having an electricalconductivity above a specified value.

In view of the foregoing discussion, it can be appreciated that there isa substantial unfulfilled commercial need for an efficient andeconomical petroleum recovery method applicable to oil formationscontaining high salinity and/or high divalent ion concentration.

SUMMARY OF THE INVENTION

The present invention concerns a petroleum recovery process usable informations containing water having high salinities, e.g. total dissolvedsolids in excess of 5000 parts per million and/or high concentrations ofdivalent ions such as calcium and/or magnesium e.g. greater than about200 parts per million. The surfactant system comprises at least twosurfactants:

1. a mixture of two or more primary anionic surfactants, specifically ablend of two or more organic sulfonates such as petroleum sulfonate or asynthetic alkyl or alkylaryl sulfonate at least one of the organicsulfonate anionic surfactant differs from the other organic sulfonate inthat it's average equivalent weight is from 10 to 60 percent less andpreferably from 35 to 55 percent less than the first organic sulfonate.

2. a solubilizing co-surfactant which renders the primary anionicsurfactant soluble in the particular high salinity and/or high divalention concentration formation water, and which may be any one of thefollowing surfactants or mixtures thereof.

a. a non-ionic surfactant such as an ethoxylated aliphatic compound oran ethoxylated alkylaryl compound

b. a non-ionic mercaptain-related surfactant such as an ethoxylatedalkyl or alkylaryl thiol;

c. an alkyl or alkylaryl polyalkoxy alkyl sulfonate compound having thefollowing structure:

    RO(CH.sub.2 CH.sub.2 O).sub.n R'SO.sub.3.sup.- M.sup.+

wherein R is an alkyl or alkylaryl group having from 8 to 20 carbonatoms,

n is an integer from 1 to 20

R' is ethyl, propyl or hydroxypropyl,

SO₃ represents the sulfonate radical, and

M⁺ is a monovalent cation such as sodium, potassium or ammonium; and

d. an alkyl or alkylaryl polyethoxy sulfate surfactant having thefollowing general structure:

    RO(CH.sub.2 CH.sub.2 O).sub.n SO.sub.3.sup.- M.sup.+

wherein R, n and M+ have the same meaning as in (c) above.

Among other factors, the ratio of the two or more organic sulfonates andthe choice of solubilizing co-surfactant are influenced by formationwater salinity, divalent ion concentration, and formation temperature.

At least one of the organic sulfonates should be predominantly watersoluble and have an average equivalent weight less than 400 andpreferably less than 350, while at least one other of the organicsulfonates should be at least partially oil soluble and preferably partoil soluble and part water soluble. The average equivalent weight of thesecond organic sulfonate should be greater than 400 and preferablygreater than 450, and the average equivalent weight should also be lessthan 600 and preferably less than 550.

There are many different petroleum sulfonates available commercially,differing from one another in their average equivalent weight as well asin the range and distribution of equivalent weight. Whenever as many asthree different petroleum sulfonates are available, there are threepossible combinations of two materials, and the essence of our inventionconcerns a method for identifying the optimum combination, both as toselection of components and weight ratio of the components, which resultin the maximum oil recovery and requires the least possible amount ofsolubilizing co-surfactant for efficient operation in any particularformation water being used for the tests.

The particular organic sulfonates to use in making the blend, as well asthe weight percentages of the two or more anionic organic sulfonateprimary surfactants and the solubilizing co-surfactant, are carefullychosen so the surfactants are slightly soluble or exhibit borderlinesolubility using the minimum amount of solubilizing co-surfactants inthe particular field water in which the surfactants are to be employed,which preferably has about the same salinity and hardness as theformation water present in the petroleum formation into which thesurfactant fluid is to be injected. The ratio of the two or more organicsulfonates is adjusted so as to require the minimum quantity ofsolubilizing co-surfactant to be mixed therewith to achieve the desiredcondition of borderline solubility. This ratio results in excellent oilrecovery and ensures the minimum chemical cost.

The concentrations of surfactants which produce the desired condition ofborderline solubility are determined by preparing a series of samplescontaining various concentrations of the two or more primary anionicsurfactants and the solubulizing co-surfactant dissolved in actualsamples of formation or field water to be employed in the field project,and measuring the electrical conductivity of the samples. The electricalconductivity is plotted as a function of the ratio, and the point on theelectrical conductivity vs. concentration ratio curve having a minimumvalue and/or an inflection point is identified. The concentration ratiocorresponding to this inflection and/or minimum point is the ratioyielding borderline solubility of the multi-component surfactantcomposition in that particular brine and is the preferred concentrationratio to be employed in the oil recovery process.

By preparing several mixtures of organic sulfonates and determining theminimum amount of solubilizing co-surfactant to achieve borderlinesolubility for each blend of organic sulfonates, the blend requiring theminimum amount of solubilizing co-surfactant may be identified. Thisblend is then used for surfactant flooding oil recovery in the formationfor which the study was made.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the percent of solubilizer required for two blends oftwo different petroleum sulfonate samples required to achieve borderlinesolubility showing how two blends of different materials requiresignificantly different amounts of solubilizing co-surfactant eventhough the equivalent weights of the two blends are equal.

FIG. 2 illustrates the oil recovery efficiencies for a series of runsusing different blends of petroleum sulfonate and the amount ofsolubilizing co-surfactant required to achieve a condition of borderlinesolubility for each blend.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Our oil recovery process involves a carefully balanced multi-componentsurfactant system which will be comprised of the following types ofsurfactants.

1. a blend of at least two primary anionic surfactants, preferably anorganic sulfonate such as a petroleum sulfonate, or a synthetic alkyl oralkylaryl sulfonate; and

2. A solubilizing co-surfactant which may be comprised of one or more ofthe following types of compounds:

a. a non-ionic surfactant such as an ethoxylated alcohol or anethoxylated alkylaryl compound,

b. a non-ionic mercaptain-related surfactant such as an ethoxylatedalkyl or arlkylaryl thiol;

c. an alkyl or alkylaryl polyethoxy alkyl sulfonate having the followingformula:

    RO(CH.sub.2 CH.sub.2 O).sub.n R'SO.sub.3.sup.- M.sup.+

wherein R is an alkyl or alkylaryl group having from 8 to 20 carbonatoms, in the alkyl chain,

n is an integer from 1 to 20

R' is ethyl, propyl or hydroxypropyl,

SO₃ represents the sulfonate radical, and

M⁺ is a monovalent cation such as sodium, potassium or ammonium; and

d. an alkyl or alkylaryl polyethoxy sulfate surfactant having thefollowing general formula:

    RO(CH.sub.2 CH.sub.2 O) SO.sub.3.sup.- M.sup.+

wherein R, n and M⁺ have the same meaning as in (c) above.

Petroleum sulfonate is a particularly desirable primary surfactant foroil recovery purposes because it is readily available, comparativelyinexpensive and quite effective under certain conditions for recoveringpetroleum from subterranean, petroleum-containing formations. Petroleumsulfonate is, unfortunately, insoluble in water having salinitiesgreater than about 5000 parts per million total dissolved solids, and/ormore than about 500 parts per million divalent ions which are generallycalcium and/or magnesium. If a normally water soluble petroleumsulfonate is added to a brine having greater salinity and/or divalention concentration than the above-identified limits, the petroleumsulfonate is insoluble and will precipitate and ultimately settle out ofthe solution, forming a layer usually under the aqueous solution. Ifsuch a fluid were injected into a subterranean, permeable oil formation,little interfacial tension reduction would be accomplished because thepetroleum sulfonate is not soluble in the aqueous fluid in which it isinjected; moreover, there is a considerable probability that plugging ofat least some of the small capillary flow channels in the oil formationwould occur. Accordingly, either a different surfactant must beutilized, which is at least slightly soluble in the formation water inwhich the fluid is to be injected, or another material must be added tothe surfactant fluid which will have the effect of increasing thesolubility of the primary anionic surfactants, e.g., petroleum sulfonateor other organic sulfonate in the presence of the high salinity anddivalent ion-containing formation water. Alcohols are sometimes employedfor this purpose, although they have only limited effectiveness and,additionally, it is preferable to utilize a material which is itself asurface active agent and so is capable of reducing the interfacialtension between the formation petroleum and the injected drive water.

Any of the above-identified four general classes of solubilizingco-surfactants may be combined with organic sulfonates such as petroleumsulfonate, and when a proper ratio is achieved between the concentrationof the organic sulfonate and the solubilizing co-surfactant, the organicsulfonate is rendered soluble in the presence of high salinity and/orhigh divalent ion-containing formation water and so can effectivelyreduce the interfacial tension between oil and water and thereby recoversubstantial amounts of oil from a formation through which the aqueoussurfactant solution is passed.

The choice of solubilizing co-surfactant is influenced by the formationwater salinity and divalent ion concentration and by the formationtemperature. The ethoxylated alcohols and thiols are effective up tosalinities of about 20,000 to 50,000 parts per million total dissolvedsolids and in formations whose temperatures are as high as 100°-150° F.The alkyl or alkylaryl polyethoxy sulfates are effective in highersalinities, up to 200,000 parts per million, but hydrolyze attemperatures above 150° F. The alkyl or alkylaryl or polyethoxyalkylsulfonates are tolerant of both very high salinities and hightemperatures.

We have found that the degree of solubility of the surfactantcomposition in the field water is extremely critical to the oil recoveryefficiency in the process. If the surfactant is much more soluble inwater than oil, then the surfactants tends to be distributed throughoutthe bulk of the water phase including both formation water and injecteddrive water, and little effectiveness will be achieved at theinterfacial zones between oil and water. Similarly, if the surfactant issubstantially more soluble in oil than it is in water, the surfactantwill partition into and distribute itself throughout the oil phase, andwill have little effect on the surface tension existing at theinterfacial zone between oil and water. The optimum surfactanteffectiveness is achieved if there is a condition of borderlinesolubility of the surfactant fluid in the drive water and/or formationwater, so the surfactants tend to exist in higher concentrations at theinterfacial zone between oil and water than in either the oil phase orthe water phase.

We have found that when using blends of organic sulfonates such aspetroleum sulfonates and one or more solubilizing co-surfactants such asthose enumerated above, the optimum oil recovery efficiency occurs whenthe concentrations of the materials are carefully balanced so as toproduce a condition of borderline solubility. If too little solubilizingco-surfactant is used in combinationwith the primary anionic organicsulfonates, not all of the primary surfactants are rendered soluble andat least a portion thereof will precipitate in the aqueous solution.This can, as discussed above, result in at least reducing theeffectiveness of the surfactant fluid for the purpose of recovering oil,and may lead to permanent, irreversible damage to permeability of theformation matrix, which will prevent any further displacement ofpetroleum from the formation. On the other hand, if more than theminimum amount of solubilizing co-surfactant which achieves theconditions which we have described above as borderline solubility isused in combination with the primary anionic blended organic sulfonatesurfactants, the surfactants are rendered too soluble in the aqueousphase and the amount of oil displaced by such a solution being injectedinto a formation is reduced fairly substantially. Moreover, since thecost of the solubilizing co-surfactants is generally from two to fivetimes the cost of the primary anionic organic sulfonate surfactant, theresult of using too much solubilizing co-surfactant is that the fluidcost is increased and the amount of oil recovered by the use of thefluid is decreased, with rapidly diminishing economic attractiveness ofthe process.

The amount of solubilizing co-surfactant necessary to achieve theabove-described desired condition of borderline solubility is highlydependent on all of the possible variations in the structuralcharacteristics of the surfactant molecules employed. The averageequivalent weight of the anionic primary organic sulfonate surfactant,for example will affect the amount of solubilizing co-surfactantrequired to achieve the condition of borderline solubility. In theinstance of using alkyl or alkylaryl polyethoxy sulfates or sulfonatesas solubilizing co-surfactants, any change in the length of the alkylchain which comprises the hydrophobe of the surfactant molecule, or achange in the number of ethylene oxide groups condensed with themolecule, will change the amount of that solubilizing co-surfactantneeded to achieve the condition of borderline solubility with whateverprimary anionic surfactant or mixture thereof it is used. Furthermore,the aqueous fluid salinity and the concentration of divalent ionspresent in the fluid will also vary the amount of the surfactants neededto achieve borderline solubility. Generally, higher salinity and/orhigher concentrations of divalent ions of the aqueous fluid is which thesurfactants are dissolved require increasing numbers of ethylene oxideunits to be present on the solubilizing co-surfactant molecule.

We have found that the only satisfactory method for determining theproper concentrations of primary anionic surfactant and solubilizingco-surfactant involves actually preparing a series of solutionscontaining the materials being considered for use in a particularapplication in various concentrations, and determining the ratio ofanionic primary surfactant to solubilizing co-surfactant which producesthe desired condition of borderline solubility in the particularenvironment of salinity and hardness in which the surfactants are to beemployed in a surfactant flood. It is highly desirable that thesurfactant fluid salinity and concentration of divalent ions match thesalinity and divalent ion concentration of the formation water asclosely as possible, so the surfactants can be tailored to operate in anoptimum fashion in that particular aqueous environment.

As a starting point, at least 3 and preferably at least 5 differentsolutions should be prepared for each blend of two or more organicsulfonates, e.g., petroleum sulfonate, samples to be tested. The totalconcentration of the blends should be held constant at a value of about1-2 percent and the concentration of solubilizing co-surfactant variedfrom about 0.1 to 1.0 percent. Stated another way, the totalconcentrations should vary between 1 and 3 percent and the weight ratioof solubilizing co-surfactant to primary surfactant blend should bevaried between 0.1 and 1.0.

Having compared the series of surfactants in the formation water asdescribed above, the minimum ratio of solubilizing co-surfactant to eachprimary anionic surfactant blend which results in the desired conditionof borderline solubility is determined by either of several procedures.

The samples can be mixed thoroughly and allowed to stand for at leastseveral hours and preferably overnight. Samples containing insufficientsolubilizing co-surfactant will separate into two distinct phases; arelatively clear aqueous phase and a separate surfactant-oil phase.Depending on the particular surfactants used, the salinity, and otherfactors, the clear phase may be on the top or bottom. The first samplein the series (i.e., the sample having the least amount of solubilizingco-surfactant) which does not exhibit two distinct phases is the samplecorresponding to borderline solubility. A second or more series of suchtests may be made to define the conditions of borderline solubility moreprecisely. In another method, the samples are placed in a suitable celland the electrical conductivities of each of the samples are determined.The conductivity is then expressed as a function of the ratio of theconcentration and preferably the function is represented graphically. Insome instances a sharp minimum value will be identified; whereas inother cases the conductivity function will exhibit a clearly identifiedinflection point, but the sign of the slope will not necessarily change.In still other situations, an inflection point will occur first as thesolubilizer concentration is increased and somewhat later a minimumvalue will be identified, in which case the inflection point identifiesthe preferred value. The ratio of surfactants which result in theminimum conductivity or in the occurrence of the first inflection pointof the conductivity function, is the ratio which will produce thecondition of borderline solubility in the aqueous surfactant fluid, andis also the ratio of surfactants which we have found will achieve theoptimum oil recovery in a formation containing water having the salinityand hardness similar to that utilized in the tests.

We have discovered that by preparing a number of samples of blendedorganic sulfonates such as petroleum sulfonates for example, whichorganic sulfonates have different average equivalent weights anddifferent equivalent weight ranges and distributions and determining theamount of any preselected solubilizing co-surfactant required to reachthe condition of borderline solubility for various ratios of suchblended materials, it is possible to identify a preferred blend oforganic sulfonates which will achieve the maximum possible oil recoveryat the conditions of the test using the least amount of solubilizingco-surfactant. Since the cost of the preferred solubilizingco-surfactants is from three to five times the cost of petroleumsulfonates, the economics are highly favorable when such minimumsolublizer-containing fluids are used for a surfactant flooding oilrecovery process. The most cost-effective organic sulfonates for use informing the blend to be used in a particular application are identifiedby determining which blend requires the minimum amount of solubilizingco-surfactant to achieve a condition of borderline solubility in thefield water to be employed in the application, or in an aqueous fluidhaving about the same salinity and divalent ion concentration the fieldwater to be employed in the particular application. Generally,commercially available samples of petroleum sulfonates havecharacteristic average equivalent weight values and ranges of equivalentweights which are relatively constant from one batch to another andwhich are determined by the hydrocarbon feed stocks used to prepare thepetroleum sulfonates as well as by the manufacturing processes employed.There are many commercially available products from which the two ormore products to blend together may be chosen. Some products arepredominantly water soluble, some are predominantly oil soluble and somehave varying amounts of oil soluble and water soluble components. Whileit is taught in the prior art to mix a water soluble and an oil solublepetroleum sulfonate to form a blend which is more effective for oilrecovery purposes than either product alone, there are many possibleblends of different oil soluble and water soluble petroleum sulfonatespossible, some of which produce good results and some of which producepoor results in our process. Moreover, a blend of two particularpetroleum sulfonates which yield excellent results in one applicationmay produce poor results in another having significantly differentformation water salinity, divalent ion concentration, etc. Finally,different blends may be found which produce equivalent oil recoveryefficiencies under a particular set of test conditions but which requiresignificantly different amounts of solubilizing co-surfactant, whichcauses one system to be much more costly than another.

The method of our invention is best understood by referring to theattached drawing, in which FIG. 1 illustrates the results of a series oftests using various blends of three petroleum sulfonate samples andindicates the amount of solubilizing co-surfactant required to achieve acondition of borderline solubility for each blend in the particularconditions of these tests. In all of the tests on which the curves ofFIGS. 1 and 2 were run, the salinity of the water used to prepare thesolutions was 110,000 parts per million total dissolved solids includingapproximately 7600 parts per million divalent ions, principally calciumand magnesium. The solubilizing co-surfactant was a sulfonated, 5 molepolyethoxylated nonyl phenol. Three petroleum sulfonates were used toprepare the two series of blends, designated as A, B and C. The averageand ranges equivalent weights of each of these materials was as follows:

                  T A B L E I                                                     ______________________________________                                        PETROLEUM    AVERAGE       RANGE OF                                           SULFONATE    EQUIVALENT    EQUIVALENT                                         SAMPLE       WEIGHT        WEIGHTS                                            ______________________________________                                        A (WITCO TRS 40)                                                                           335           273-440                                            B (WITCO TRS 10-80)                                                                        413           250-464                                            C (WITCO TRS 18)                                                                           495           353-640                                            ______________________________________                                    

Curve 1 of FIG. 1 represents a series of blends of samples A and B. Therelative amounts of A and B was varied and the average equivalent weightof the blend was determined.

Curve 2 of FIG. 1 similarly represents a blend of samples A and C.

For each point on each curve, a series of samples were prepared using2.0 weight percent of the petroleum sulfonate blend and varying theconcentration of the solubilizing co-surfactant from about 0.05 to about2.5, and the concentration of solubilizing co-surfactant needed toachieve borderline solubility for each blend was determined usingelectrical conductivity measurements according the method describedabove. This procedure was repeated for at least four blends of A and Band for at least four blends of A and C.

It is possible to prepare a blend of A and B or a blend of A and Chaving any desired average equivalent weight between about 335 and 413.For example, dashed line 3 of FIG. 1 represents an average equivalentweight of 375 and a blend of A and B having this average equivalentweight is found at point 4 where curve 1 intersects line 3. A blend of Aand C exists at point 5 where line 3 intersects curve 2 having preciselythe same average equivalent weight. It can be seen that while theaverage equivalent weights are the same for these two blends, the amountof solubilizing co-surfactant required for borderline solubility is notthe same, since 1.25 percent solubilizer is required for blend 4 whileonly 0.72% solubilizer is required for borderline solubility of the sameconcentration of blend 5. Since the concentration of petroleum sulfonateis the same in blends 4 and 5 and blend 4 requires 42% less solubilizingco-surfactant, the cost per pound of which is about five times the costof petroleum sulfonate, it is apparent that blend 5 is much moreeconomical than blend 4.

The oil recovery efficiency cannot be determined directly from FIG. 1and it is necessary to perform another series of tests to identify theparticular blend of petroleum sulfonate which, when used in combinationwith the proper amount of solubilizing co-surfactant as determinedabove, will accomplish the maximum oil recovery in the particular fieldbrine being tested. A series of oil displacement tests in cores or sandpacks is performed using several different blends of organic sulfonatewith the amount of solubilizing co-surfactant determined to be optimumfor each individual blend as described above. Two series of runs wereperformed in this manner, one using various ratios of petroleumsulfonates A and B and another using various ratios of petroleumsulfonate samples A and C. The oil recovery efficiencies are depicted inFIG. 2 for both series of runs, with curve 6 representing oil recoveryof a mixture of A and B with the ratio of A and B varied to produceblended samples having average equivalent weights from about 350 toabout 450. The maximum oil recovery was obtained using a blend of A andB having an equivalent weight of about 370. Curve 7 relates the oilrecovery efficiency of blends of A and C to the average equivalentweight of the blends, and the maximum oil recovery is realized using ablend of A and C having an average equivalent weight of about 380.

It can be seen from FIG. 2 that the oil recovery achieved using theoptimum blend of A and B is only slightly superior to that attainedusing the optimum blend of A and C, and the average equivalent weight ofthe optimum A and B blend was about the same as the average equivalentweight of the optimum A + C blend. Moreover, the concentration of theA + C blend was exactly the same as the concentration of the A + B blendand the test conditions were the same. The most significant differencein the oil recovery results was that blend A + B required 1.25 percentsolubilizing co-surfactant and blend A + C required only 0.72 percent ofthe same solubilizer. The 42% reduction in solubilizing co-surfactantrequired to solubilize blend A + C is very significant since the cost ofsolubilizing co-surfactant is much greater than the cost of petroleumsulfonates.

The results attained in the foregoing example are generally typical ofthe results obtained using the process of our invention in designingblended petroleum sulfonate plus alkyl or alkylaryl polyethoxy sulfonatesolubilizers for use in highly saline and/or hard water environments.Optimum results are generally obtained using blends of petroleumsulfonates or other organic sulfonates in which at least one sample is apredominantly water soluble petroleum sulfonate, having an averageequivalent weight less than 400 and preferably less than about 350 andat least one other petroleum sulfonate is at least partially oil solubleand has an equivalent weight greater than 400 and preferably greaterthan 450, but less than 600 and preferably less than 550.

It is very important to realize that both B and C are at least partlysoluble while sample A is essentially all water soluble. Thus theparticular blends of A + B and A + C tested for oil recovery are both amixture of a water soluble and an oil soluble petroleum sulfonate, andthe two blends have nearly the same average equivalent weight, yet theamount of solubilizing co-surfactant required to solubilize the sameweight percent of the one of the two blends in the same brine is 42%greater than the amount required to solubilize the other blend.

Once the optimum blend and solubilizing co-surfactant are identified, athird series of oil recovery tests in cores or sand packs should beperformed in which the total surfactant concentration is varied and theoil recovery efficiency as a function of total surfactant concentrationis determined. For example, if it is determined that the optimum ratioof solubilizing co-surfactant to blended primary surfactant for aparticular application is 0.4 in a series of tests using 2.0 percentpetroleum sulfonate and 0.8 percent solubilizing surfactant (2.8 percenttotal surfactant concentration), oil recovery efficiency may be measuredfor surfactant fluids containing 2.0, 2.5, 3.0, and 3.5 percent totalsurfactant at the same concentration ratio to identify to totalconcentration which results in the maximum oil recovery.

Once the optimum blend, total concentration of surfactants and weightratio of solubilizing co-surfactant to blended primary surfactant areidentified as described above, the field procedure is similar to fieldpractices commonly used for surfactant flooding operations. No freshwater preflush is ordinarily needed since the surfactants are tailoredto operate optimally at the salinity and divalent ion concentration ofthe formation water. Sacrificial agents may be used to reduce surfactantadsorption if the particular formation being exploited adsorbs thesurfactants to be used.

The surfactant fluid is preferably prepared in formation water or fieldwater having a salinity and divalent ion concentration about equal tothe formation water. The quantity of surfactant fluid utilized willgenerally be from 0.1 to 1.0 pore volume based on the pore volume offormation to be swept by the surfactant fluid. The surfactant fluidshould be followed by injection of a mobility buffer, which is anaqueous solution of a hydrophilic, viscosity increasing polymer such aspolyacrylamide or polysaccharide. Generally from 50 to 1000 parts permillion polymer concentration is sufficient to produce a fluid having aviscosity greater than the formation petroleum viscosity, which isadequate to ensure efficient displacement. From 0.1 to 0.5 pore volumesof the viscous mobility buffer solution is used. This is in turnfollowed by injection of field water to displace all of the injectedfluids and petroleum through the formations to the production well.Field water injection is continued until the oil cut of the producedfluid drops to an uneconomic level.

Thus, we have disclosed how the best blend of two or more organicsulfonates may be identified, and the optimum ratio of solubilizingco-surfactant to the blend of anionic surfactant such as a blend of atleast two organic sulfonates samples, including petroleum sulfonates,may be determined in simple laboratory tests that are relatively quickand inexpensive to perform.

While some discussion of the mechanism and theory of operation of ourinvention has been included in the foregoing discussion, it was includedonly for the purpose of additional disclosure and it is not necessarilymeant to imply that these are the only or even the primary mechanismsresponsible for the proper functioning of our invention. Although wehave disclosed our invention in terms of a number of illustrativeembodiments, our invention is clearly not so limited since manyvariations thereof will be apparent to persons skilled in the art ofenhanced oil recovery without departing from the true spirit and scopeof our invention, and it is our desire and intention that our inventionbe limited and restricted only by those limitations and restrictionswhich appear in the claims appended immediately hereinafter below.

We claim:
 1. In a method of recovering petroleum from a subterranean,petroleum-containing, permeable formation said formation also containingwater of known or determinable salinity greater than 5000 parts permillion total dissolved solids, said formation being penetrated by atleast one injection well and by at least one production well, both wellsbeing in fluid communication with the petroleum formation, comprisinginjecting an aqueous, saline surfactant-containing fluid into theformation via the injection well to displace and drive petroleum towardthe production well from which it is recovered to the surface of theearth, said surfactant fluid comprising an aqueous fluid whose salinityis about equal to the salinity of the formation water and havingdissolved therein surfactants comprising a mixture of at least twoorganic sulfonate primary anionic surfactants and at least onesolubilizing co-surfactant, wherein the improvement comprises:blendingat least two anionic organic sulfonate surfactants at least one of whichis predominantly water soluble and at least one which is at leastpartially oil soluble to form a blend which requires a minimum amount ofsolubilizing co-surfactant to achieve a condition of borderlinesolubility
 2. A method as recited in claim 1 wherein the organicsulfonate which is predominantly water soluble has an average equivalentweight less than
 400. 3. A method as recited in claim 1 wherein theorganic sulfonate which is predominantly water soluble has an equivalentweight less than about
 350. 4. A method as recited in claim 1 whereinthe organic sulfonate which is at least partially oil soluble has anaverage equivalent weight of at least 400 and not greater than
 600. 5. Amethod as recited in claim 1 wherein the organic sulfonate which is atleast partially oil soluble has an average equivalent weight of at least450 and not greater than
 550. 6. A method as recited in claim 1 whereinthe solubilizing co-surfactant is selected from the group consisting ofethoxylated aliphatic compounds, ethoxylated alkylaryl compounds, alkylor alkylarylpolyethoxy sulfates, alkyl or aliphatic polyethoxyalkylsulfonates, alkylarylpolyethoxyalkyl sulfonates and mixtures thereof. 7.A method as recited in claim 6 wherein the solubilizing co-surfactant isan alkyl or alkylaryl polyethoxy sulfate.
 8. A method as recited inclaim 6 wherein the solubilizing co-surfactant is an alkyl oralkylarylpolyethoxyalkyl sulfate having the following structure:

    RO(CH.sub.2 CH.sub.2 0).sub.n SO.sub.3.sup.- M.sup.+

wherein R is an alkyl or alkylaryl group having from 8 to 20 carbonatoms in the alkyl chain, n is an integer from 1 to 20, and M⁺ is amonovalent cation.
 9. A method as recited in claim 6 wherein thesolubilizing co-surfactant is an alkyl or alkyarylpolyethoxyalkylsulfonate.
 10. A method as recited in claim 9 wherein the solubilizingco-surfactant is an alkyl or alkylarylpolyethoxyalkyl sulfonate havingthe following structure:

    RO(CH.sub.2 CH.sub.2 O).sub.n R'SO.sub.3.sup.- M.sup.+

wherein R is an alkyl or alkylaryl group having from 8 to 20 carbonatoms in the alkyl chain, n is an integer from 1 to 20, R' is ethyl,propyl or hydroxypropyl, and M⁺ is a monovalent cation.
 11. A method asrecited in claim 1 wherein the blend of organic sulfonate surfactantswhich require the minimum amount of solubilizing co-surfactant isdetermined by preparing at least two series of at least 4 samples eachfrom at least three organic sulfonate samples having different averageequivalent weights, each sample having a total surfactant concentrationbetween 1.0 and 4.0 percent and weight ratios of solubilizingco-surfactant to the blend of organic sulfonate surfactants of eachseries being varied from 0.1 to 1.0, measuring the electricalconductivities of the samples and identifying the ratios correspondingto the sample which produced the minimum electrical conductivities ineach series and identifying the blend of organic sulfonates whichrequire the minimum amount of solubilizing co-surfactant to achieve thecondition of borderline solubility.
 12. A method as recited in claim 11comprising the additional steps of forming a graphical representation ofelectrical conductivity versus weight ratio of solubilizingco-surfactant to the blended primary anionic surfactant and identifyingthe minimum value of electrical conductivity on the graphicalrepresentation and the weight ratio of surfactants correspondingthereto.
 13. A method of recovering petroleum from a subterranean,petroleum-containing permeable formation penetrated by at least twowells in fluid communication therewith, said formation containing waterhaving a known or determinable salinity of at least 5000 parts permillion total dissolved solids, comprising:introducing into theformation via one of said wells, an aqueous saline fluid having asalinity about equal to the salinity of the formation water, saidsurfactant fluid comprising a solubilizing co-surfactant and a blend ofat least two organic sulfonates one of which is predominantly watersoluble and one of which is at least partially oil soluble, the blend oftwo or more organic sulfonates being chosen from a group of at least twoblends of at least three organic sulfonates having different averageequivalent weights, the selected blend requiring less of thesolubilizing co-surfactant than the other blends to achieve a conditionof borderline solubility in the saline fluid.
 14. A method as recited inclaim 13 wherein the organic sulfonates are independently selected fromthe group consisting of petroleum sulfonate, alkyl sulfonate, alkylarylsulfonates, and mixtures thereof.
 15. A method as recited in claim 14wherein the blend of organic sulfonates is a blend of petroleumsulfonates.
 16. A method as recited in claim 13 wherein the solubilizingco-surfactant is selected from the group consisting of ethoxylated alkylor alkylaryl compounds, alkyl or alkylaryl polyethoxy sulfates, alkyl oralkylaryl polyethoxyalkyl sulfonates, and mixtures thereof.
 17. A methodas recited in claim 16 wherein the solubilizing co-surfactant is analkyl or alkylarylpolyethoxyalkyl sulfonate.
 18. A method of recoveringpetroleum from a subterranean, petroleum-containing permeable formation,said formation containing water of known or determinable salinity anddivalent ion concentration, said formation being penetrated by at leastone injection well and one production well, both wells in fluidcommunication with the formation, comprising:a. preparing a first seriesof at least three samples comprising a first blend of two organicsulfonates in different weight ratios, one of the organic sulfonatesbeing at least partially water soluble and one other of the organicsulfonates being at least partially oil soluble, the organic sulfonateshaving average equivalent weights which differ by at least 50; b.determining the minimum weight ratio of a preselected solubilizingco-surfactant selected from the group consisting of ethoxylated alkyl oralkylaryl compounds, alkyl or alkylarylpolyethoxy sulfates alkyl oralkylarylpolyethoxyalkyl sulfonates and mixtures thereof, required tosolubilize each sample of the first series of blends of organicsulfonates in an aqueous fluid having a salinity about equal to thesalinity of the formation water; c. preparing a second series of atleast three samples of two organic sulfonates in different weightratios, one of the organic sulfonates being at least partly watersoluble and one other of the organic sulfonates being at least partlyoil soluble, the average equivalent weights of the organic sulfonatesdiffering by at least 50, at least one of the organic sulfonates of thesecond series having an average equivalent weight which differs from thecorresponding organic sulfonate in the first series by at least 25; d.determining the minimum weight ratio of the same solubilizngco-surfactant as used in step (b) required to solubilize each sample ofthe second series of blends of organic sulfonates in an aqueous fluidhaving about the same salinity as was used in step (b); e. determiningwhich series of blends of organic sulfonates require the lesser weightratio of solubilizing co-surfactant for solubility in aqueous fluidhaving about the same salinity as the formation water; f. determiningthe concentration and weight ratio of the series of blended organicsulfonates identified in step (e) when used with the minimum weightratio of solubilizing co-surfactant required for solubility at thesalinity of the formation water which result in optimum oil recovery; g.preparing an aqueous saline surfactant-containing fluid comprising thepreselected solubilizing co-surfactant and the organic sulfonate blendidentified in step (e) above in the concentration and weight ratiodetermined in step (f) above having a salinity about equal to thesalinity of the formation water; h. injecting the aqueous salinesurfactant-containing fluid of step (f) into the petroleum containingformation via the injection well to displace the formation petroleumtoward the production well; and i. recovering petroleum displaced by thesurfactant-containing fluid from the formation via the production well.19. A method as recited in claim 18 wherein at least one of the organicsulfonates in selected from the group consisting of petroleum sulfonate,aliphatic sulfonate, alkyl sulfonate, alkylaryl sulfonates and mixturesthereof.
 20. A method as recited in claim 18 wherein the organicsulfonate which is at least partially water soluble used in the blend ofstep (g) has an average equivalent weight less than
 400. 21. A method asrecited in claim 20 wherein the organic sulfonate has an averageequivalent weight less than about
 350. 22. A method as recited in claim18 wherein the organic sulfonate which is at least partly oil solublepresent in the blend of step (g) has an average equivalent weight of atleast 400 and not greater than
 600. 23. A method as recited in claim 22wherein the average equivalent weight of the organic sulfonate is atleast 450 and not greater than 550.